2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/93359Lecture Notes for the Les Houches Summer School LXIII on Quantum Fluctuations in July 1995 to appear in Elsevier Science Publishers B.V. 1997, edited by E. Giacobino and S. Reynaud.57 pages (LaTeX 2.09) including 11 figures (.ps)Quantum Physicstext